Title   Physical Condition for the Gas in Galactic Centers
Pi     E. Schinnerer
Time   240 hrs

1. Name of program and authors
1.5.6: Name -- Physical Condition for the Gas in Galactic Centers
       Authors: E. Schinnerer, D.S. Meier


2. Science goal:
   Molecular gas is abundant in the central kiloparsec of nearby star
   forming galaxies. Generally, it is believed that large-scale
   structures such as stellar bars and/or spiral arms cause this
   pile-up of gas in the galactic centers which might then be
   transformed into stars. However, it is not clear what physical
   conditions are responsible for the excitation of the molecular gas
   seen in its 12co(1-0) line emission. By combining observations of
   molecules such as, e.g., HCN, CS, HCO+, CH3OH, HNC, CCH, N2H+, SiO,
   and HC3N one can probe if the physical environment is dominated by
   kinematic shocks (due to stellar bars causing the gas to move
   inward), embedded star formation (still invisible in the optical to
   NIR wavelengths) or photon dominated regions (caused by already
   developed HII regions). Gaining insight into the actual physical
   state of the molecular gas is of great importance as the star
   forming process in the galactic centers must proceed under very
   different conditions compared to the disk: the interstellar
   radiation field becomes stronger while the dynamical timescales
   become much shorter (< 1e7 yr). Angular resolution on the GMCs
   scale is paramount to avoid confusion between the molecular cloud
   complexes and the more diffuse extended gas. The high spectral
   resolution is required to resolve shock fronts that are expected to
   be present in barred galaxies.

3. Number of sources: ~ 5 nearby (barred) starburst galaxies
   
4. Coordinates:

4.1. random over sky, out to ~ 10-15 Mpc

4.2. Moving target: no 

4.3. Time critical: no


5. Spatial scales:

5.1. Angular resolution (arcsec): 0.05" 

5.2. Range of spatial scales/FOV (arcsec): 0.05" --> 20"
     single-field

5.3. Required pointing accuracy: 0.5"


6. Observational setup

6.1. Single dish total power data: required

     Observing modes for single dish total power: 
     on-the-fly mapping

6.2. Stand-alone ACA: no

6.3. Cross-correlation of 7m ACA and 12m baseline-ALMA antennas: 
     required

6.4. Subarrays of 12m baseline-ALMA antennas: no


7. Frequencies:

7.1. Receiver band: Band 3

7.2. Lines and Frequencies (GHz): 

     multi-line 
     3 frequency settings in Band 3

7.3. Spectral resolution (km/s): 5 km/s

7.4. Bandwidth or spectral coverage (km/s or GHz): 
      ~ 600 km/s per line with a maximum bandwidth of 8 GHz for 
      most multiplexing freedom
                                        
8. Continuum flux density:

8.1. Typical value (Jy):

8.2. Required continuum rms (Jy or K):

8.3. Dynamic range within image:

8.4. Calibration requirements: absolute      ( 1-3% / 5% / 10% / n/a )
                               repeatability ( 1-3% / 5% / 10% / n/a )
                               relative      ( 1-3% / 5% / 10% / n/a )


9. Line intensity:

9.1. Typical value (K or Jy): 1.25 - 2.5 mJy

9.2. Required rms per channel (K or Jy):  0.1 mJy

9.3. Spectral dynamic range: 5 - 30

9.4. Calibration requirements: absolute      ( 5% )
                               repeatability ( 5% )
                               relative      ( 5% )


10. Polarization: no

11. Integration time for each observing mode/receiver setting (hr):
    1 track (+/- 2hr)  x 5 sources x 4 configurations x 3 frequency settings


12. Total integration time for program (hr): 240 hr


13. Comments on observing strategy : 

    Short spacings are important for good imaging fidelity as well as
    to trace the distribution of the various molecules probing different
    physical conditions such as photon dominated regions, shocks and
    cold dense clouds.
    
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Comments:

this proposal requires cross correlations of 7m ACA and 12 m
baseline-ALMA short spacings are indeed very important for this
project

they propose to do 5 sources without any justification for that
number.
R.:  Given our experience from local nearby galactic nuclei, it is clear
    that the ongoing and/or recent star formation history in the
    centers will likely have an impact of the molecular gas properties
    in form of consumption of gas, ionization, mechanical impact
    etc. In addition, the exact bar properties (only a large-scale or
    a nuclear bar, double barred, strong bar vs. oval, etc.) will also
    shape the dynamical and physical properties of the molecular
    gas. Thus it is paramount to have a large enough sample to
    discriminate general trends from unusual but very localized conditions.

Huge amount of time, but very interesting science..